1. Technical Field
The present invention relates to solid-state imaging devices having high image quality and low power consumption.
2. Related Art
As solid-state imaging devices mounted in cellular phones, digital cameras, etc., there are a charge-coupled device (CCD) image sensor (hereinafter called a CCD sensor) and a CMOS image sensor (hereinafter called a CMOS sensor).
A CCD sensor achieves a correlation double sampling (CDS) function for removing noises and a so-called global electronic shutter function for shooting images of rapidly moving objects without distortion. The global electronic shutter function is a function for eliminating the distortion of images of moving objects by storing light-generated charge at a time for a number of light-receiving elements that are placed two-dimensionally. Therefore, a CCD sensor generally has an advantage of excellent image quality. At the same time, however, a CCD sensor has disadvantages of high driving voltage and high power consumption.
On the other hand, a CMOS sensor generally has an advantage that the process cost is low with low power consumption due to low driving voltage. However, a general CMOS sensor cannot achieve both of the global electronic shutter function and the CDS function at the same time.
For example, in a CMOS sensor of CMOS-active pixel sensor (APS) type having a four-transistor configuration, a noise component is read out first by resetting a floating diffusion, which serves as a charge-retaining region, for each reading line and then a signal component is read out, which is the CDS function. That is, in the CDS function, since signal read-out is performed immediately after noise read-out, light-generated charge generated by a photodiode is transferred to the floating diffusion via a transfer transistor immediately after noise read-out. Since read-out is performed line by line, the transfer of light-generated charge to the floating diffusion is also performed line by line. Therefore, the storage period differs line by line.
In addition, there is another driving method that employs the global electronic shutter in which the light-generated charge of all pixels is stored at a time using photodiodes and transferred at a time to floating diffusions via transfer transistors. In this case, however, light-generated charge is stored in the floating diffusions of all lines. Therefore, signal read-out needs to be performed before noise read-out. In other words, the device needs to be driven in the order of signal read-out, reset, and noise read-out for each line. Such a method may slightly degrade image quality because there is no correlation between the noise included in the signal that is read out and the noise that is generated in noise read-out. Also in this case, there is another method that the noises of all pixels are read out before transfer, which, however, requires a frame memory for retaining the noises that are read out.
As a solution to the above disadvantages, there is an example of related art, which will be described later, that discloses a technique to concurrently start/end the signal-storing operation of all pixels. In the related art example, a charge-retaining region is provided directly under a transfer gate. Thus, signal charge generated from a photodiode is temporarily stored in the charge-retaining region and then transferred to a floating diffusion. By this method, the global electronic shutter function is achieved.
Japanese Unexamined Patent Publication No. 2002-368201 is an example of related art.
By the way, if an extremely intense light enters into a photodiode, the generated amount of light-generated charge increases and may cause overflow from the photodiode. Such overflow charge (hereinafter also called excessive charge) flows from a photodiode-forming region into a floating diffusion via a transfer transistor so as to be discharged when the floating diffusion is reset.
In the proposal of Japanese Unexamined Patent Publication No. 2002-369201, however, the excessive charge from the photodiode is stored in the charge-retaining region provided directly under the transfer gate. This means that the light-generated charge of the subsequent frame flows into the charge-retaining region, which induces a degradation of image quality.